Rapid escalation of energy costs in recent years has spurred efforts to improve efficiency in plant operations. Distillation practices have received great attention, since the process of distillation, the predominant unit operation in the oil refining and chemical industries, consumes prodigious amounts of energy.
One energy-saving procedure is the practice of multiple stage, e.g., two-stage, condensation of the overhead vapors from a distillation column. Multi-stage condensation procedures allow energy savings by permitting the use of higher temperatures at which heat may be removed from the overhead vapor. Such a process is described, for example, in U.S. Pat. No. 3,320,159. In at least one two-stage procedure for condensation of distillation column overhead vapors, the first stage of condensation is operated under conditions to condense just enough liquid to provide reflux for the distillation column, and the second stage of condensation is operated under conditions to provide the top product of the column. For two-stage condensation to be of value in heat recovery, the vapor or top product from the distillation column must be a multi-component mixture, and there must be a reasonable temperature spread between its dew point and bubble point, e.g. 20 degrees F. Most hydrocarbon distillations fit this requirement.
A very common commercial process for which two-stage condensation can be helpful is the separation of gasoline boiling range materials (often 400.degree. to 430.degree. F. ASTM end point) as the top product from a feed containing also kerosene, jet fuel, distillate fuel and gas oil components. As now practiced, however, two-stage condensation procedures have the disadvantages of added capital cost and increased complexity. In general, the process, as known, has required the use of two accumulators, with interconnecting piping, pumps, etc. Accordingly, a need has existed for a process which would provide the advantages of two-stage condensation without the disadvantages mentioned. The present invention satisfies that need.